Human herpesvirus 6B (HHV-6B) reactivation is a recognized event following allogeneic hematopoietic stem cell transplantation (HSCT). However, its impact on engraftment kinetics and post-transplant outcomes remains incompletely defined, particularly in the era of letermovir prophylaxis, when the use of antivirals with anti-HHV-6B activity, such as ganciclovir and valganciclovir, has declined. To determine the incidence of HHV-6B reactivation and assess its impact on post-transplant outcomes, including time to platelet and neutrophil engraftment, lymphocyte recovery, and transplant-related complications. We conducted a prospective cohort study of adults who underwent allogeneic HSCT between March 1 and December 1, 2022. Plasma HHV-6B DNAemia was monitored serially during the first 100 days post-transplant, with clinical follow-up for 6 mo and extended follow-up to 2 yr for mortality and relapse outcomes. In this cohort of 102 patients (49 females [48%]; median age, 58 yr [IQR, 48 to 66]), HHV-6B reactivation occurred in 50 patients (49%), with a median onset of 28 days post-transplant (IQR, 22 to 31) and a median peak viral load of 684 IU/mL (IQR, 72.7 to 3500). Compared with patients without DNAemia, those with HHV-6B reactivation had delayed platelet engraftment (26 days [IQR 18 to 34] versus 19 days [IQR 17 to 26]; P = .025) and longer time to achieve an absolute lymphocyte count (ALC) >300 & times; 10(6)/L (29 days [IQR 25 to 40] versus 26 days [IQR 22 to 32]; P = .04). Persistent HHV-6B DNAemia (n = 21, 20.6%), defined as >1000 IU/mL for two consecutive weeks, was associated with further delays in platelet (30 days [IQR 19 to 39] versus 19 days [IQR 17 to 26]; P = .01), ALC (33 days [IQR 24 to 42] versus 26 days [IQR 22 to 32]; P < .05), and neutrophil engraftment (22 days [IQR 19 to 24] versus 20 days [IQR 17 to 22]; P = .05). In adjusted analysis, HHV-6B DNAemia remained an independent predictor of relapse (aHR, 2.21; 95% CI, 1.00 to 4.85; P = .04). In competing risk analysis, persistent HHV-6B DNAemia was significantly associated with relapse (8/21 [39.5%; 95% CI, 18.0 to 60.0] versus 9/52 [17.0%; 95% CI, 8.0 to 28.0]; P = .04). HHV-6B reactivation was frequent after allogeneic HSCT and was associated with delayed hematopoietic recovery and an increased risk of disease relapse, particularly among patients with persistent DNAemia. These findings highlight the clinical significance of HHV-6B in post-transplant outcomes and underscore the need for prospective studies to refine monitoring and management strategies.